Prince William walks with his head bowed behind the coffin of his mother Diana, Princess of Wales draped in the Royal Standard, on its way to London's Westminster Abbey for the funeral ceremony, 06 September. (ULLI MICHEL/AFP/Getty Images)

In this handout photo, HRH Prince William is pictured on a commemorative stamp, in honour of his 21st birthday. It is the first time a royal birthday is being celebrated with the issuing of commemorative stamps and coins. (Getty Images)

Kate Middleton and Prince William on the day of their graduation ceremony at St Andrew's University in St Andrew's on June 23, 2005 in Scotland. (Photo by the Middleton Family/Clarence House via GettyImages)

Prince William and Kate Middleton pose for photographs in the State Apartments of St James Palace on November 16, 2010 in London, England. Clarence House announced the engagement of Prince William to Kate Middleton. (Chris Jackson/Getty Images)

TRH Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge smile following their marriage at Westminster Abbey on April 29, 2011 in London, England. (Photo by Chris Jackson/Getty Images)

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge attend day 1 of Royal Ascot 2017 at Ascot Racecourse on June 20, 2017 in Ascot, England. John Phillips/Getty Images for Ascot Racecourse

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge pose for the official tour portrait for their trip to Canada and California in the Garden's of Clarence House on June 3, 2011. (Chris Jackson / Getty Images for St James's Palace)

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge pose for a photograph with their son, Prince George Alexander Louis of Cambridge on August 2013 in Bucklebury, Berkshire. (Michael Middleton/Kensington Palace via Getty Images)

Catherine, Duchess of Cambridge holds Prince George as he and Prince William, Duke of Cambridge's look on while visiting an exhibition at the Natural History Museum on July 2, 2014 in London, England.(John Stillwell - WPA Pool/Getty Images)

Prince William, Duke of Cambridge wears a traditional Japanese King's Costume (Samurai) during a visit to the set of a historical drama at NHK Public Broadcasting Studios in Tokyo, Japan. (Photo by Chris Jackson/Getty Images)

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge, with their children, Princess Charlotte and Prince George, enjoy a short private skiing break on March 3, 2016 in the French Alps, France.(John Stillwell - WPA Pool/Getty Images)

President Barack Obama talks with the Prince William, as Catherine, Duchess of Cambridge plays with Prince George at Kensington Palace on April 22, 2016 in London, England. (Pete Souza/The White House via Getty Images)

The royal family stands on the balcony during the Trooping the Colour, this year marking the Queen's 90th birthday at The Mall on June 11, 2016 in London, England. (Photo by Ben A. Pruchnie/Getty Images)

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge, depart The Lindo Wing with their newborn son Prince George of Cambridge at St Mary's Hospital on July 23, 2013 in London, England. Chris Jackson/Getty Images

Prince William, Duke of Cambridge holds a bird of prey during his visit at Anglesey agricultural show on his first official engagement since the birth of his son Prince George of Cambridge. Chris Jackson/Getty Images

Prince William, Duke of Cambridge holds Prince George of Cambridge as Catherine, Duchess of Cambridge gives him a commemorative card during a visit to the Bilby Enclosure at Taronga Zoo. Chris Jackson/Getty Images

Prince William, Duke of Cambridge and Prince George of Cambridge arrive at the Lindo Wing after Catherine, Duchess of Cambridge gave birth to a baby girl at St Mary's Hospital on May 2, 2015 in London, England. (Photo by Chris Jackson/Getty Images)